Attenuators are circuits that attenuate (reduce) an input signal by a predefined amount or percentage. Limiters are circuits that are designed to limit input power to an amplifier or other device to prevent damage to the device. Limiters are used in a variety of applications, in particular for front end protection of receivers. Some limiters do not attenuate the input signal at all until a threshold limit is reached. Linearizers are circuits that compensate for some non-linear behavior of another circuit, such as an amplifier, to provide a linear response. All three types of circuits may be used in RF systems, including microwave systems.
Microwave attenuators often are useful in microwave systems for a variety of reasons. These include buffering poor return losses and reducing system gain levels. Simple resistive attenuators can be realized from well-known design equations. Examples are the “T-network (“Tee”) and Pi-network (“Pi”) attenuators.
FIG. 1A is a schematic 10 of conventional Tee and Pi attenuators. Limiting power POUT is determined by the series resistor in the Tee case. In the Pi case, POUT is primarily determined by the series resistor; however, there is a small amount of current that passes through the shunt resistor. The design equations for a T- and Pi-network resistive attenuators are well known. For a T-network they are,
                                          R            1                    =                                    Z              0                        ⁢                                                            10                                      dB                    /                    20                                                  -                1                                                              10                                      dB                    /                    20                                                  +                1                                                    ,                            (        1        )                                                      R            2                    =                      2            ⁢                          Z              0                        ⁢                                          10                                  dB                  /                  20                                                                              10                                      dB                    /                    10                                                  -                1                                                    ,                            (        2        )            where R1 and R2 are the series and shunt resistors respectively, Z0 is typically 50Ω, and dB is the desired attenuation value in dB.
For minimum attenuation, R2→∞ while R1→0. The T-network attenuator has similar properties with the series resistor approaching 0Ω and the shunt resistors approaching infinity for small attenuations. Due to process limitations in the Qorvo 0.25 μm Gallium Nitride (GaN) process, the minimum dimension length of a resistor is 5 μm. This effectively puts a limit on the minimum attenuation that a GaN resistor network can have in order to limit at a particular power level. To explain more clearly, a limiter is desired to have a small insertion loss, especially at higher powers where additional tenths of dB in power may be very costly. To approach 0Ω for the series resistors, R1, the implemented GaN resistor width must be very wide or the length must approach 0. However, making the resistor wider increases the saturation current thereby increasing the power the attenuator would limit at. The minimum attenuation value versus limiting power is derived in the following paragraphs. Again, the limiting power is defined as the maximum output power from the attenuator (assuming a 50 Ω system). The input power at that maximum output power will be larger than the output power plus the nominal attenuation. This is due to compression of the nonlinear resistors.
The peak current through a resistor isIpk=Isatw,  (3)where w is the width of the resistor (perpendicular to current flow), and Isat for the process used is 1 A/mm. The resistance of a sheet resistor is given by
                              R          =                                    l              w                        ⁢                          R              s                                      ,                            (        4        )            where l is the length (parallel to current flow) of the resistor and RS is the resistance in Ω per square. The peak current through the resistor will determine the power that can be passed to the output of the attenuator.
                              I          pk                =                              I            sat                    ⁢                      l            R                    ⁢                      R            s                                              (        5        )            
For the T-network attenuator, the limiting resistor is the second series resistor assuming the physical size of the first series resistor is much larger so that there is no significant limiting effect. The limiting power is easily found as
                              P          out          T                =                                            1              2                        ⁢                          I              out              2                        ⁢                          Z              0                                =                                                    1                2                            ⁢                              I                pk                2                            ⁢                              Z                0                                      =                                                            Z                  0                                2                            ⁢                                                                    (                                                                                            lR                          s                                                ⁢                                                  I                          sat                                                                                            R                        1                                                              )                                    2                                .                                                                        (        6        )            where Z0 is the system impedance. R1 can be found by the design equations for the T-network resistive attenuator.
For the Pi-network, there is a small adjustment due to the current that passes through the shunt resistor. The output current is related to the current through the series resistor as
                              I          out                =                                            R              1                                                      R                1                            +                              Z                0                                              ⁢                      I                          R              ⁢                                                          ⁢              2                                                          (        7        )            and so the limiting power is
                              P          out          Pi                =                                            Z              0                        2                    ⁢                                    (                                                                    lR                    s                                    ⁢                                      I                    sat                                                                    R                  2                                            )                        2                    ⁢                                                    (                                                      R                    1                                                                              R                      1                                        +                                          Z                      0                                                                      )                            2                        .                                              (        8        )            A similar equation can found for the T-network if the first series resistor is the limiting component.
FIG. 1B is a graph illustrating the limiting power versus attenuation for T- and Pi-network resistive attenuators with the minimum length of l=5 μm and a saturation current of lsat=1 A/mm. One may note that the Pi-network allows lower attenuation values for a given limiting power. Any point above the line (l=5 μm) is realizable in the GaN process used.
Microwave amplifiers typically have maximum input power ratings. These ratings are derived based on laboratory testing that determines the maximum amount of input power the amplifier can withstand prior to the onset of damage or permanent degradation. Users of the amplifier must be careful to not provide an input power greater than this recommended level. A well-known two-port microwave circuit is a power limiter. At non-limiting levels of input power, the output power of the limiter is very nearly the input power. However, as the level of input power rises beyond a critical threshold, the limiter begins to limit the amount of output power. The remaining power is either reflected back to the input port or dissipated within the circuit in some manner.
Microwave limiters are often realized in a technology that is not integrated in the same process as a Monolithic Microwave Integrated Circuit (MMIC) amplifier. For instance, limiters using PIN diodes—which have a wide, un-doped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor—are not available for fabrication on currently available Gallium Arsenide (GaAs) or Gallium Nitride (GaN) MMIC processes. Additionally, many types of limiters require Direct Current (DC) bias (power) to provide limiting with very high levels of input power.
Thus, there is a need for attenuators, limiters, and linearizers that can be integrated with MMICs. By putting these circuits on the same die with each other parts counts are reduced, interconnect costs and problems are reduced, reliability is improved, process mismatch problems are eliminated, and economies of scale may be achieved.